The Bet HaLevi wrote a pamphlet on the Mitzvah of Bitachon- a feeling in the heart of total reliance on Hashem. There he says that Bitachon could only come after a person has engrained in himself the Emunah that nobody in this world has any say over any matter other than Hashem. A person cannot give a benefit or take away something from another individual, if it wasn't decreed by Hashem first. When we sit down in a meeting to try to make a sale, we have to know, the buyer has absolutely no say on whether or not we are going to make money there. It is only up to Hashem. The buyer is just a puppet. If somebody made a decision that caused him a loss of money, that was also 'מאת ה-from Hashem.
Our financial position is in His hands. Our free will is limited to areas that have to do with Averot and Mitzvot. A person cannot receive something, even one moment before he is decreed to have it. And nobody could delay giving him something that he's decreed to have now. If someone is wondering why didn't he get a raise yet, and he's thinking, "Maybe the boss forgot or maybe he's not noticing me," that could be true, but it's only because the time for the raise did not yet arrive, so Hashem didn't bring it about. The moment the person is supposed to get a raise, the boss will remember, or the boss will take notice.
The same is true in every area. There is only one Boss in this world, and He has all the say. If a person is being harmed by another individual, and he's trying every which way to free himself from that harm, if he doesn't also ask Hashem to stop the harm from coming, says the Bet HaLevi, he's like an individual who is getting beaten with a rod, and he's pleading with the rod to stop hurting him, never once asking the one who's holding the rod to stop.
Everything that happens to us is a manifestation of Hashem's will. How fortunate are we that Hashem is always doing everything for the best. When a person knows this, he'll feel at ease wherever he is, because he knows he's in Hashem's hands.
At the same time, we're obligated to make our own efforts and do what is necessary in the normal way of the world to obtain our needs. Hashem controls everything, but he put the obligation of Hishtadlut upon us. As the Chovot HaLevavot writes in the Shaar HaBitachon (Ch. 4) man is obligated to take care of his food, drink, and clothing according to his needs; he shouldn't throw that on Hashem and say, "if it is decreed for me to have food then I'll have it."The reason is because Hashem told us that's our part-to live in this world in a natural way. It has to look like we're the ones making the money. But the entire time we have to believe that it has nothing to do with us.
There was a man who refused to take medicine for his weakened condition saying he's a Ba'al Bitachon and doesn't need medicine. The Chazon Ish told him, "You're obligated to take care of your health the same way you would do for your friend. Overcome your will now and listen to the doctors. That is your Mitzvah."
When a person knows that he is just going through the motions, and the results are out of his control, he'll be able to do everything with a sense of calm, knowing he's just making an effort because Hashem wants him to. He'll have less second guessing, less regret and less heartache when he doesn't see the results that he wanted. He'll say, "This is what Hashem wants, so I'm happy with it." Furthermore, when he knows Hashem is in full control, he won't fear people, he'll never do something wrong.
The Midrash says in Devarim Rabba (Ch. 4) that Hashem tells us, "Listen to Me, do My will; nobody ever listened to Me and lost as a result." When someone knows he only has one boss to please, he'll be a man of principle. And this knowledge, says the Bet HaLevi, is the stepping-stone for having true Bitachon in Hashem.